It is known to provide seals between moving and stationary components. In this respect FIG. 1 shows two examples of previously-proposed seal assemblies comprising labyrinth seals 30′, 30″ disposed between a first component 10′, 10″, which may rotate with respect to a second component 20′, 20″. The first and second components are typical of rotating and stationary components found in a gas turbine, for example a stator shroud between adjacent rotors. The examples show the geometry of the cavity downstream of a last seal fin, including the shape of the platforms which the air flows between. However, there is typically a leakage flow 50′, 50″ through the labyrinth seal 30′, 30″ and this leakage flow may return to a mainstream flow 60′, 60″. The first example comprises a large volume cavity 40′, which may be adequate to dissipate a leakage jet from the labyrinth seal 30′. In contrast, the second example comprises a small volume cavity 40″, which has little available space to dissipate a leakage jet from the labyrinth seal 30″. A small cavity 40″ may be required where other factors, such as the shroud geometry, do not permit a large cavity 40′.
Previous seal assembly designs have assumed that the static pressure in the cavity 40′, 40″ was broadly constant and that the energy of the leakage jet 50′, 50″ through the seal 30′, 30″ had dissipated prior to entering the mainstream flow-field 60′, 60″. This was generally true since most prior art sealing assemblies comprised relatively large volume cavity. However, in the case of a small cavity, there may not be enough space for the jet 50″ to dissipate prior to entering the mainstream flow 60″.
Furthermore, although it was previously accepted that a high energy jet would flow over the seal fin tips, the path of the jet through the cavity was unknown and there was no appreciation of the flow structure set up within the cavity. In addition the small cavity geometry leads to a spoiling of the mainstream flow.
As shown in FIG. 2 (which corresponds to the second example described above), the previously-proposed small cavity 40″ design results in the high energy jet entering the mainstream flow 60″ in a non-preferential direction, thereby resulting in a spoiling of the mainstream flow. The prior design shown in FIG. 2 allows a large captive vortex 70″ to be formed by the counter-rotating vortex within the cavity 40″. The vortex 70″ directs the flow radially into the mainstream flow 60″, thereby disturbing the mainstream flow. This disturbance may have a negative impact on the flow efficiency and hence the efficiency of a device in which the seal assembly operates.
Furthermore, in the case of small cavity 40″, the labyrinth seal 30″ overall radius is high relative to the mainstream flow 60″. With the larger radius of the labyrinth seal 30″, it is more difficult to control seal clearances due to the higher metal expansion and contraction rates. Higher leakage flow rates may therefore result for the small cavity example, thereby further exacerbating the problem.
A further example of a prior art seal assembly is shown in FIG. 3. A seal 80 may be provided between a blocker door 82 and a fan casing 84a, 84b in a thrust reverser unit 90 for a jet engine. However, steps and gaps between the fan casing 84b and blocker door 82 may generate uncontrolled vortices 86, which shed downstream and separate the mainstream flow 60 from the annulus wall. The shed vortices and/or the flow separation may have a negative impact on the flow efficiency.
The present disclosure therefore seeks to address these issues.